Ten female rhesus monkeys, in early stages of pregnancy, were studied to determine the effects of a dietary supplement of genistein, a phytoestrogen found in soy, on the steroidogenesis in the maternal-fetoplacental unit. The monkeys were placed in two equal groups. The experimental group received 8mg/kg body weight of genistein each Monday through Friday. Dosing continued until day 155 of gestation, at which time a Cesarean section was performed to collect the fetus and placenta. Blood was collected from the maternal peripheral circulation twice weekly during the study period. At delivery blood was collected from the maternal peripheral circulation, the uterine veins, the ovarian veins, and the fetal heart. Placental tissues were collected and frozen until assayed. Sera from the blood sample were frozen until assayed for steroid hormones. Estradiol levels in the genistein-treated monkeys were higher than in the control monkeys during gestation and at delivery all maternal and fetal blood levels from the genistein-treated monkeys were significantly higher. The estrone levels in the genistein-treated monkeys trended to be higher than the control but were not significant (p=0.057). There were no significant differences between the two groups in weight gained during pregnancy, fetal weights, or placental weights. There were trends but no significant differences in blood levels of DHEA-S, or progesterone. The placental assays found no significant differences in placental LDL and HDL levels. These data suggest that the elevated levels of estradiol in the maternal blood may be due to deconjugation of estrone in the gut and conversion to estradiol. The higher levels in the fetal blood indicate that the genistein crossed the placental barrier and stimulated the production of estradiol, since estradiol produced by the placenta is preferentially directed to the maternal circulation, as a protective measure to the fetus. This finding is significant when the use of soy-based food products used by the mother during pregnancy and soy-based infant formulas is increasing. Reports in the literature suggest that soy-based formulas may be associated with an increased incidence of goiter and thyroid disease. FUNDING Chancellor?s Grant, Tulane University School of Medicine